Matt Yglesias had a great short post on why passenger-miles-travelled is a poor metric for measuring the greenness of a mass transit system:
The whole idea of trying to talk about which city’s mass transit system is greenest in terms of emissions per passenger-mile is terribly flawed.
Just think of it in terms of cars. Driving 5 miles in a 20 mpg car takes a quarter of a gallon of gasoline. Driving 25 miles in a 45 mpg car takes over half a gallon. Being the guy with the 5 mile commute and the 20 mpg car is considerably greener than being the guy driving much further in his Prius. The point of intra-urban transportation networks—whether you’re talking about the mass transit element or the private cars or bicycles or whatever—isn’t to transport people arbitrary distances, it’s to get people where they’re going. Having trips that aren’t very long isn’t cheating, it’s a great way to achieve efficiency.
Of course, greenness isn’t the only useful measure of a transportation system. I’ll just add two points on top of what Matt has said.
1) I’d go a step further and say it’s actually a greener transportation system in general that allows people to make shorter trips. Shorter car trips mean less time on the road, which itself means less congestion, less fuel consumed and less wear on the roads. Shorter bus trips mean less time on the bus, less fuel consumed (though the difference is very small), less wear on the buses and more room for others. I’m not saying it’s bad to take long trips, I’m just saying a transportation system that would allow everyone to walk everywhere they could possibly need to go would be the greenest of all.
2) While we’re on the topic of terrible greenness measures, miles-per-gallon is actually a rather poor way of comparing fuel efficiency of different vehicles. Fuel consumed is actually inversely proportional to mpg, so the slope of the fuel consumed graph is very steep at low values, and very flat at high values. This means that there’s a lot of savings from going from, say, 5 MPG to 10 MPG, but much less savings going from 50 to 100. Imagine if you had two cars, one getting 10 MPG and another getting 20. Supposed you drove 100 miles per week on each. With the two, you consume 15 gallons of gas. If you wanted to get an average of 20 MPG, you could replace the 10 MPG car with one getting twenty and consume only 10 gallons per week, or you could replace the 20 MPG car with one getting 30 and consume 13 1/3 gallons per week. This would be obvious if we used gallons per 100 miles (or litres per 100km as they do in Australia). So if we want to measure the “greenness of cars” it makes sense to put the thing we care about (gallons) as the numerator, not the denominator.